3.5 Pump
As noted earlier, HPLC pumping systems have evolved more than any other component of LC. Gravity-fed open column systems were later pressurized with gas, which was replaced by syringe pumps in early HPLC, then by single piston pumps, and today with specially engineered dual reciprocating piston pumps. Today’s pumps provide constant pressure and mobile phase flow by alternating the pumping actions between two pumping systems. When either pump is in full stroke, constant flow is easily provided but as each pump reaches the end of a stroke flow rates may be altered. This is overcome in some systems by an oval-shaped caming device that speeds the pumping rate at the end of each piston stroke. At the end of the stroke, one pump speeds up the flow of mobile phase as the other pump head decreases the pumping rate. This combined action provides constant flow rates in modern HPLC systems. While the simultaneous operation of two pumps and their associated pistons, cams, and check valves are difficult to describe in words, an animation makes the process easy to understand. Such an animation is shown in Animation 3.3. The animation can be viewed by clicking on the figure below.
Animation 3.3 Illustration of a dual reciprocating piston HPLC pump.
Pulse Damper: Early HPLC pumps produced pressure, and therefore flow pulses, this was especially true with single piston pumps which required a pulse damper to be installed down-line from the pump. Today’s systems, with advanced pump design, produce little pressure fluctuations and either do not require a pulse damper or electronically compensate for the very small pressure fluctuations.
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